In modern industrial manufacturing, as the unit capacity and total machine capacity of the switching power supply are increasingly improved, these manufacturers (driving power supply, LED light, mobile phone charger, audio equipment, etc.) have higher requirements on high-frequency transformers. Leakage inductance is one of the concerns.

Leakage inductance must exist in the primary circuit of switching power supply, especially in transformer and inductor. In fact, the parameters of the leakage inductance affect the main parameters of the switching power supply to a great extent. For example, the voltage loss of 40A/5V switching power supply is up to 20%, which also affects the weight and efficiency of switching power supply. This is why the research on inductance leakage is getting more attention.

(OUGE Switching Power Transformer)

(OUGE Circuit Board)

Definition of Transformer Leakage Inductance

Leakage inductance is the coupling coefficient between the primary winding and secondary winding of the transformer. In simple terms, it means that the magnetic field lines produced by the coil cannot pass through the secondary coil all. Therefore, the inductance producing magnetic leakage is called leakage inductance.

When the leakage inductance value is small, part of the winding of the transformer hasn’t the transformation effect, but react with choke coil to produce equivalent components. If the complete coupling between the primary winding and secondary winding (the coupling coefficient k=1) becomes an ideal transformer, the leakage inductance value is zero. However, the coupling coefficient of the transformer is usually less than 1. Because it is not fully coupled, part of the winding has the function of inductance. On the equivalent circuit, leakage inductance refers to the way that the primary or secondary coil of the transformer is connected in series with choke coil.

Factors Influencing Leakage Inductance

For fixed transformers which have been made, leakage inductance is related to the following factors:

K: Widing coefficient is proportional to leakage inductance. For simple primary winding and the secondary winding, 3 is taken. But if the secondary winding is staggered with the primary winding, 0.85 is taken. That’s why it’s recommended to wrap like a sandwich, and the leakage will drop a lot, about a third less than it used to be.

Almost everyone who makes power switches knows about this winding, but few people have really studied it. I’m sure many people have eaten sandwiches, two layers of bread with a layer of cream between them. The sandwich winding method can reduce the leakage inductance of the transformer. In this way, the voltage stress of the power switch tube can be reduced and the absorption circuit can be reduced to improve the power efficiency indirectly.

However, this method will increase the coupling capacitance between the initial and secondary magnetic line. During the working process of switching power supply, the distributed capacitance of the winding is recharged and discharged repeatedly, which not only reduces the power efficiency but also forms LC oscillator with the distributed inductance of the winding to produce ringing noise.

Lmt: The length of the entire winding is averaged per turn on the bobbin. Therefore, transformer designers prefer to choose the longer core of the core column. The wider the winding, the less leakage. Controlling the winding amount to a minimum is very beneficial to reduce leakage whose effect of turns on leakage is quadratic.

(OUGE Bobbin)

Nx：The number of turns

W: Winding width

Tins: The insulation thickness of the wire

bW: Total thickness of all winding of the transformer

The Influence of Leakage Inductance

Leakage inductance is an essential indication of switching transformer, which has a significant influence on the performance index of switching power supply. When the switch device is off. The reverse electromotive force will be generated, which can easily break the switch device under overvoltage. Leakage inductance also can form an oscillation loop with the distributed capacitance in the circuit and the distributed capacitance of the transformer coil, which can make the circuit oscillate and radiate the electromagnetic energy outwards, resulting in electromagnetic interference.

The influence of leakage inductance of transformer on rectifier circuit is as follows:

When the overlap angle Y appears, the average value of the rectifier output voltage Ud is decreased, resulting in the reduction of power factor.

During operation, a gap occurs in the voltgate, which causes distortion of the grid waveform and becomes a source of interference. Orthovoltage may mislead the thyristor through, so it must be added to the absorption circuit.

It can limit the short- circuit current and reduce the di/ dt of the thyristor, which is conducive to open the thyristor safely.